1. Field of the Invention
The present invention relates to a semiconductor laser and an optical semiconductor device using the semiconductor laser and particularly to improvements in frequency characteristics and in fabrication yield of a semiconductor laser that has diffraction grating.
2. Description of the Related Art
At present, a higher transmission speed of an optical semiconductor device is desired. For example, a semiconductor laser for a wavelength-division multiplexing method is used in the optical semiconductor device using a communication light source for optical fiber communication to realize a transmission speed of 100 Gb/s that has become recently standardized. The optical semiconductor device includes a 4 channel semiconductor laser that is driven at the transmission speed of 25 Gb/s or 28 Gb/s per one channel, outputs light of different wavelengths, and realizes high capacity transmission. At present, an electroabsorption-modulated DFB-laser is used as the semiconductor laser that has the transmission speed of 25 to equal to or more than 28 Gb/s. For the optical semiconductor device, an optical transmitter and receiver using the laser element as a transmission light source has been in practical use. However, for the perspective of high-density integration into a high-speed router, there is a high demand for smaller size, lower power consumption, and lower cost in an optical communication transmitter and receiver with the speed of 100 Gb/s. Because a directly modulated laser element is simple in structure and thus is simple in implementation if high-speed operation of the directly modulated laser element can be realized, it is expected smaller size, lower power consumption, lower cost, and the like in the optical semiconductor device can be realized. In the transmission at the transmission speed of 10 Gb/s over a short distance of equal to or less than 10 km, the directly modulated laser element is already in practical use. The directly modulated laser element is a distributed feedback (hereinafter referred to as the DFB) laser element in which the structure of a cavity is DFB that oscillates with a single wavelength in a manner that makes the transmission over the distance of 10 km possible. For example, it is considered that the DFB laser element is also used in the directly modulated laser element with the high transmission speed of 25 to 28 Gb/s. Moreover, for example, a directly modulated semiconductor laser still on the drawing board, as the directly modulated laser element with the higher transmission speed, which is driven at the speed of 25 Gb/s or 28 Gb/s, is disclosed in “G. Sakiano, et al, ‘25.8 Gbps Direct Modulation AlGaInAs DFB Lasers with Ru-doped InP Buried Heterostructure for 70° C. operation,’ OFC/NFOEC 2012 Technical Digest, OTh3F.3, 2012.”
The DFB laser element has a structure in which diffraction grating is formed along an optical axis direction inside the element. Because oscillation occurs with two wavelengths in principle in the uniform diffraction grating structure, a contrivance is provided that oscillates in a single mode in a semiconductor laser with the diffraction grating. As one technology relating to the contrivance, there is a quarter-wave phase shifted (λ/4-shifted) diffraction grating structure. Furthermore, a dielectric film is coated on each of both facets of the semiconductor laser. The dielectric film in which light reflectivity on the end facet is low reflectivity is an anti-reflection film (a low reflection film) and the dielectric film in which the light reflectivity on the end facet is high reflectivity is a reflection film (a high reflection film). The semiconductor laser that has λ/4-shifteded diffraction grating structure is disclosed in JP 62-95886 A, JP 61-216383 A, JP 61-47685 A, and JP 2000-286502 A. In the semiconductor laser disclosed in the patent documents, an improvement in characteristics is made by combining an arrangement of a shift position of the λ/4-shift in the diffraction grating and a selection of dielectric films coated on the both facets of the semiconductor laser.
The semiconductor laser that has the λ/4-shifted diffraction grating structure has a problem in that the single mode cannot be maintained at the time of high optical output because optical intensity is localized in the shift position of the λ/4-shift. Therefore, the semiconductor laser that has a corrugation-pitch-modulated (CPM) diffraction grating structure as a structure for reducing the localization of the light intensity is disclosed in “Makoto Okai, ‘Spectral characteristics of distributed feedback semiconductor lasers and their improvements by corrugation-pitch-modulated structure,’ Journal of Applied Physics, Vol. 75, No. 1, pp. 1-29, 1994.” In the semiconductor laser that has the CPM diffraction grating structure, the diffraction grating has a diffraction grating region for phase shift, and λ/4-shift is spatially made in the diffraction grating region in a gradual manner.